The present invention relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear wheels of a four-wheel drive vehicle. More particularly, the present invention is directed to an improved geared coupling that is operable for varying the amount of torque transferred as a function of operating temperature and rotational speed differential applied between an input and an output.
In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for directing power (i.e., drive torque) to the non-driven wheels of the vehicle. In "part-time" power transfer systems, a transfer mechanism is incorporated into the driveline which is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. In addition, such transfer mechanisms typically include a mechanical "mode" shift mechanism which can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheels to the driven wheels for establishing a four-wheel drive mode.
Alternatively, it is known to utilize "full-time" power transfer systems for continuously directing drive torque to both sets of wheels. Typically, the transfer mechanism used in such full-time systems is equipped with an inter-axle differential for permitting speed differentiation between the front and rear axles so as to prevent potentially damaging torque build-up in the driveline. In addition, it is also common to provide a "lockout" or brake mechanism (i.e., a differential brake) for selectively or automatically inhibiting such differential action across the inter-axle differential, thereby effectively coupling the front and rear axles in a part-time four-wheel drive mode. In some systems, the lock-out mechanisms must be selectively actuated by the vehicle operator upon the vehicle encountering a low traction road surface. However, some full-time systems are equipped with a viscous coupling or an electronically-controlled clutch mechanism for automatically locking-out the inter-axle differential during certain low traction conditions.
A third type of power transfer system is commonly referred to in the industry as an "on-demand" system. In general, "on-demand" systems are operable to automatically direct power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, the "on-demand" feature includes installation of a torque transmission device, such as a viscous coupling or an electronically-controlled clutch assembly, between the driven and non-driven axles. In either case, the amount of drive torque transferred to the non-driven axle is a function of the speed differential (i.e., slip) between the driven and non-driven axles.
As is readily apparent from the foregoing, modern power transfer systems are commonly equipped with some type of multi-plate clutch or coupling apparatus for use as an automatically-controlled differential brake or an on-demand torque transmission device. As will be appreciated, the expense and complexity associated with most electronically-controlled clutch assemblies limits their application to only the most expensive four-wheel drive vehicles. Accordingly, viscous couplings have been widely used in such vehicular applications.
A characteristic of viscous couplings is that as the operating temperature of the viscous coupling increases, due to increases in ambient temperature but primarily due to heating of the viscous fluid as a result of the work imparted on it, the torque transfer characteristics of the viscous coupling changes. However, to improve vehicle handling and driveability, it is desirable to have a torque transmission device which produces a consistent "speed sensitive" increase in torque output over a range of rotational velocity differences and operating temperatures. In this regard, Muller and Witte in their paper PDS (Porsche Dynamic Slip Control Clutch) - A New Inter-Axle Coupling Device for 4WD - Cars, SAE Paper No. 880698, describe an "on-demand" coupling device (the PDS device) capable of delivering progressive torque output in response to an increasing speed differential between the driven axle and non-driven axle. In general, the PDS device is essentially a modified dual-sun planetary gear mechanism that is operably associated with a centrifugal brake. The PDS device has an input sun gear coupled for rotation with the driven axle and an output sun gear coupled for rotation with the non-driven axle. Under normal driving conditions when there is little or no difference in the rotational velocity between the axles, the PDS device does not transfer torque to the non-driven wheels. That is, the vehicle is operating in a two-wheel drive mode. However, when the driven axle slips causing a difference between the driven and non-driven axle rotational velocities, a planet carrier is caused to rotate at relatively high velocity. However, this rotation is resisted by the centrifugal brake that is coupled to the planet carrier and which is adapted to engage a braking surface fixed to a stationary wall portion of the power transfer device housing. With the planet carrier thus restrained, torque is progressively transferred "on-demand" via the planet gears from the input sun gear to the output sun gear for delivering torque to the non-driven wheels. One disadvantage associated with the PDS device is its use of the centrifugal brake to control the torque transfer. More specifically, by incorporating a centrifugal brake the PDS device requires modification to the power transfer device housing, the introduction of additional parts and assembly steps to the power transfer device, and the potential for increased maintenance when the centrifugal brake friction elements begin to wear. An additional disadvantage of the centrifugal brake is that in reacting the torque into the housing, the PDS device is generally incompatible with anti-lock braking systems (ABS).